Frequently in drilling operations it is desirable to perforate the tubing string within the well casing to allow well fluids to flow into the tubing string. In some instances it is desirable to penetrate the tubing string with an insert having a known internal bore so that precise calculation of well flow rates can be made.
In order to penetrate the tubing string and embed an insert therein, explosive perforating guns such as that disclosed in U.S. Pat. No. 3,199,287 have been used. U.S. Pat. No. 3,199,287 disclosed a drive wedge 40 actuated by an explosive charge. The drive wedge mechanically contacted the piston and propelled the piston through an opening in the tool and against the tubing to be perforated. The explosive gases then continued to propel the drive wedge until it came in contact with a stop wedge 60 whereupon the explosive gases slowly vented through various unsealed passages within the tool. Due to the sliding and highly loaded contact between the drive wedge and the piston, the drive wedge had to be stopped by a stop wedge before the explosive gases could reach the piston bore. If the drive wedge had not been stopped by the stop wedge while the drive wedge was still at the position shown in FIG. 1 of the patent, the drive wedge could have become jammed in the barrel when the cylindrical portion of the drive wedge encountered the piston. Thus, metal deformation would occur on the drive wedge and cause the wedge to jam itself in the bore on the far side of the piston bore.
In order to prevent that phenomenon, a flat was provided on the side of the wedge near its upper end. Such flat assured that the surface contact between the piston and the drive wedge always remained in a line contact, rather than a point contact which would have resulted if an arcuate surface on the wedge contacted a beveled flat surface on the piston. Because the drive wedge required the use of such flat, an effective seal was maintained between the drive wedge and the barrel by providing the upper section of the drive wedge with a full radius. Since the part of the drive wedge that had the full radius could not be allowed to cross the piston's bore because of the potential jamming problems, the drive wedge was required to be stopped by the stop wedge so that the recess between the flat and the full radius of the drive wedge would be positioned at the piston bore. This intermediate section, having a reduced diameter, was positioned opposite the piston bore and allowed the piston to come back into the tool after it rebounded from the tubing wall. That construction had a disadvantage in that the drive wedge was extremely costly to fabricate and piston retraction was limited by the thickness of the drive wedge at such intermediate section. Finally, stopping the drive wedge with the intermediate section aligned with the piston bore was critical, if the piston was to be allowed to retract into the tool. If a misalignment occurred, the piston could not retract into tool, and significant damage might result to the tool in attempting to extricate it from the tubing.
Also, since the drive wedge was explosively driven into the stop wedge, on occasions, it became difficult to pry apart the drive wedge from the stop wedge so that that tool could be reloaded.
In many applications, it is desirable to aim the perforating tool so that the perforation occurs and the insert is embedded in the tubing wall adjacent the location having the maximum annular cross-sectional area between the tubing string and the casing. In a deviated well, the most likely position for the maximum annular space is likely to be on top of the tubing. In order to insure that the insert is placed in this particular location, the piston bore must be positioned in the pipe so that it always faces upwardly in a deviated hole. Prior designs had runners welded to thin sections of tubing that slid over the barrel of the perforator. These two runners acting as spacers to allow the tool to be used in different sizes of tubing, would not let the tool roll within the tubing.
The perforating tool of the present invention incorporates features for overcoming some of the limitations of prior tools. In the perforating tool of the present invention, hydraulic forces built up due to the setting off of the explosive charge propel the piston from the barrel. It is only if additional force is needed to perforate the tubing that the drive wedge makes a mechanical contact with the piston. The drive wedge is driven completely past the piston bore so that after firing, the piston may fall all of the way back into the barrel. The piston has a longitudinal opening therethrough so that hydraulic pressure is equalized to either side of the piston within the barrel. Vent openings are selectively placed to allow the lubricating fluid to escape as well as to allow the explosive gases to escape, thereby avoiding the necessity of having to use the pressure of the explosive gases to drive the drive wedge into contact with the stop wedge. A piston adapter segment is connected to the top side of the piston so that if for some reason the piston does not fully retract into the barrel, the adapter segment can be sheared off and the tool removed from the tubing. An adapter segment is further designed to accommodate various sized inserts, thereby eliminating the need to have individual piston designs for each insert. The perforating tool of the present invention by using rounded counterweights and spacers having a flat surface machined thereon in combination with a belly spring, allows the tool to roll within the tubing until the desired depth is reached. The belly spring forces the flat portions of the spacers against the tubing surface to stabilize the tool before it is fired. These and other improvements can be appreciated from a further review of the specification and the drawings.